Internal combustion engine variable tuned exhaust system

ABSTRACT

An exhaust system for an internal combustion engine having an exhaust port and an exhaust chamber communicating between the ports includes a variable dimension element in the exhaust chamber adapted to vary an effective dimension of the exhaust chamber during operation of the engine.

TECHNICAL FIELD

This invention relates to improvements in internal combustion engines.In particular, this invention relates to exhaust systems for internalcombustion engines and to internal combustion engines incorporating suchexhaust systems, as well as to a method of operating such exhaustsystems.

BACKGROUND OF THE INVENTION AND PRIOR ART

A conventional internal combustion engine repeats a cycle of actions ineither two or four movements of a piston. The cylinder in a two-strokeengine, for example, has a transfer port and an exhaust port, each ofwhich is opened and closed by reciprocation of the piston. The cylinderin a four-stroke engine, for example, has, instead of the transfer portand the exhaust port, an inlet valve and an outlet valve, each of whichis also opened and closed by reciprocation of the piston.

For the sake of simplicity, that stroke of the piston, which results inthe exhaust products being scavenged from the cylinder, will hereinafterbe referred to as the exhaust stroke. Further, those means through whichthe fuel mixture enters the cylinder, being the transfer port of atwo-stroke engine or the inlet valve of a four-stroke engine, forexample, will hereinafter be referred to as the transfer port.

Furthermore, those means through which the exhaust products leave thecylinder and enter the exhaust system, being the exhaust port of atwo-stroke engine or the outlet valve of a four-stroke engine, forexample, will hereinafter be referred to as the exhaust port. It shouldbe noted that the exhaust port may further include one or more headerpipes.

After the fuel mixture has entered the cylinder through the transferport and has been compressed and ignited, the exhaust products arescavenged from the cylinder during the exhaust stroke, through theexhaust port.

It is known that the tuning or adjustment of the exhaust system of anengine can affect engine performance. It is possible to tune an exhaustsystem with regard to the opening of the exhaust port to enhance engineperformance.

For example, Australian Patent No. 584087 describes the tuning of anexhaust system of a two-stroke engine in order to establish a lowpressure at the exhaust port, when the exhaust port is open and beforethe transfer port is closed, and a higher pressure at the exhaust port,after the transfer port is closed and before the exhaust port is closed.

The purpose of establishing the low pressure at the exhaust port beforethe transfer port is closed is to enhance scavenging of the exhaustproducts from the cylinder. The purpose of establishing the higherpressure at the exhaust port after the transfer port is closed is tominimise loss of fresh fuel mixture from the cylinder. Tuning in thismanner can produce a "super-charging" effect, giving both greater enginepower and improved fuel economy.

In Australian Patent No. 584097, the tuning of the exhaust port is saidto be achieved by controlling the temperature of the gas in the exhaustsystem.

In tuning the exhaust system of a four-stroke engine, a "super-charging"effect may be achieved during valve overlap time when scavenging of theexhaust products from the cylinder is enhanced by negative pressurecreated at the exhaust port.

It has now been found that excellent tuning of the exhaust system of anengine may be achieved by providing an exhaust chamber which is variablein effective length during use.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide an improved exhaustsystem suitable for certain internal combustion engines.

Accordingly, the invention provides an exhaust system for a four strokeinternal combustion engine having at least one cylinder and at least oneheader pipe receiving exhaust gases from the at least one cylinder, theexhaust system comprising:

an exhaust chamber communicating with the at least one header pipe andan exit port, whereby exhaust gases flow from the at least one headerpipe, through the exhaust chamber and to the exit port; and

an inner member mounted at least partially within the exhaust chamber,said inner member movable in response to engine speed to vary therestriction provided to flow of gas through the exhaust chamber.

In the case of multi-cylinder four-stroke engines, which usually have aplurality of header pipes, the exhaust system of the inventionpreferably has a plurality of inner members adapted to fit with aplurality of the corresponding header pipes.

Preferably, the exhaust chamber has a centre portion and oppositeportions, one being tapered, the exit port being located in the taperedend portion, and the or each header pipe leading into the exhaust portbeing located at the end position opposite the tapered end portion.

In a particularly preferred embodiment, there are four inner memberslocated within the exhaust chamber.

Each inner member is adapted to be slidably received within or on anopposite header pipe. The inner members may further be joined togetherat a base. The inner members may be supported within the exhaust chamberby any suitable means and may be caused to move by a suitableservo-mechanism in response to engine speed.

In another embodiment of the invention, only one inner member is locatedwithin the exhaust chamber. The inner member of this embodiment may besimilar in format to the inner member described above.

It is preferred that each inner member has one end shaped to fit acorresponding header pipe, while the other end echoes the shape of allor part of the tapered end portion of the exhaust chamber. The shape ofthe inner member(s) can be important as the relationship between theinner member(s) and the tapered end portion of the exhaust chamberprovides the change in volume of the exhaust chamber aft of the innermember(s) during operation of the engine.

The inner member(s) may intervene with the gas path (as previouslydefined) in order to change the resistance to gas flow in response toengine speed. For example, the inner member(s) may be caused to movetowards the header pipes by a servo-mechanism in response to increasedengine speed, thus reducing the resistance of the exhaust chamber to gasflow.

When engine speed decreases, the inner member(s) may be caused to moveaway from the header pipes towards the exit port, thus increasing theresistance of the exhaust chamber to gas flow.

Such intervention enhances scavenging of the exhaust products from thecylinder by creating negative pressure at the exhaust port, particularlyduring valve overlap time.

This invention also provides an internal combustion engine having acylinder, a piston movable therein, an exhaust port, a header pipe, anexhaust chamber and an exit port, an inner member located at leastpartially in the exhaust chamber and adapted to fit with the headerpipe, a gas port defined in use by the header pipe, the inner member,and the exit port, characterised in that the inner 15 member is movableto change resistance of the exhaust chamber to gas flow during operationof the engine.

Where the engine is a four-stroke engine, there is preferably aplurality of the inner members which are adapted to vary the resistanceof the exhaust chamber to gas flow so as to establish a negativepressure differential at the exhaust port during the exhaust stroke.

In another embodiment, the inner member is mounted downstream of theheader pipes, preferably partially in the exit port, and its upstreamend cooperates with the downstream end of the header pipes to vary theresistance to gas flow.

BRIEF DESCRIPTION OF THE DRAWING

By way or example only, some preferred embodiments of the presentinvention will now be described with reference to the accompanyingdrawings, wherein:

FIG. 1A is a sectional view of an embodiment of an exhaust systemaccording to the invention suitable for a four-stroke engine.

FIG. 1B is a partially exploded perspective view of the embodiment ofFIG. 1A.

FIG. 2A is a sectional view of a second embodiment of an exhaust systemaccording to the invention suitable for a four-stroke engine;

FIG. 2B is a partially exploded perspective view of the embodiment ofFIG. 2A.

DETAILED DESCRIPTION OF THE DRAWING

The exhaust system in FIGS. 1A and 1B is an embodiment of the inventionwhich is particularly suitable in connection with four-stroke engines.The exhaust system of FIGS. 1A and 1B has exhaust chamber 10 havingcenter portion 11 and tapered portion 13.

Located within tapered portion 13 of exhaust chamber 10 is inner member16, provided in a shape of a pyramid (absent its base) and havingprojection 17, inner member 16 closely echoes the shape of taperedportion 13. Header pipes 20 protrude into center portion 11 at one end21 thereof and communicate with exhaust port (not shown) at the otherend.

Inner member 16 is supported within tapered portion 13 by any suitablemeans. Inner member 16 is caused to move by a suitable servo-mechanism(not shown) in the direction of arrow 23 in response to engine speed.Thus, when engine revolutions are high, inner member 16 will be locatedwithin tapered portion 13 as shown in FIG. 1A. As revolutions decrease,the inner member 16 will move towards the position shown in dottedoutline in FIG. 1A.

Referring to FIGS. 2A and 2B, the exhaust system according to theinvention comprises exhaust chamber 20 having center portion 11 andtapered end portion 13. Tapered portion 13 communicates with an exitportion (not shown). Four header pipes 26 protrude into center portion11 at one end 21 thereof and communicate with an exhaust port (notshown) at the other end.

Located within exhaust chamber 10 are four inner members 16 being joinedtogether at base 24. Each inner member 16 is slidably received withinopposite header pipe 20 at end 21 thereof.

Each inner member 16 has one end shaped to fit header pipe 20 while theother end echoes the shape of tapered end portion 13 of exhaust chamber10.

As may be seen in FIG. 2A, wall 24 of inner member 16 can fit withinwall 26 of header pipe 20. Gap 27 is provided between the group of innermembers 16 and centre portion 11 so that wall 26 of corresponding headerpipe 20 can fit into centre portion 11.

Inner members 16 are supported within exhaust chamber 10 by any suitablemeans and are caused to move there by a suitable servo-mechanism (notshown) in response to engine speed.

Inner members 16 operate to increase aft exhaust chamber volume 28 asengine speed increase, and allow volume 28 to reach maximum as enginespeed decreases. For example, inner members 16 are caused to movetowards pipes 20 and the exhaust port (not shown) by a servo-mechanism(not shown) in response to increased engine speed, thus increasingvolume 38.

When engine speed decrease, inner members 16 are caused to move awayfrom the exhaust port towards the exit port, thus reducing volume 13.

The exhaust systems illustrated in the drawings are capable ofincreasing engine power without adversely affecting scavenging.

It is to be understood that the embodiments described in connection withthe drawings are for the purpose of illustrating the invention and arenot to be interpreted as limiting in the scope of the invention. Othervariations will be apparent to one skilled in the art and are within thescope of the invention.

I claim:
 1. An exhaust system for a four-stroke internal combustionengine having at least one cylinder and at least one header pipereceiving exhaust gases from the at least one cylinder, the exhaustsystem comprising:an exhaust chamber communicating with the at least oneheader pipe and an exit port; a hollow inner member mounted at leastpartially within the exhaust chamber; said inner member movable inresponse to engine speed to vary the restriction provided to flow of gasthrough the exhaust chamber; said exhaust chamber has a first sectionand an intermediate section extending from the first section to the exitport and tapering in a downstream direction, and the inner member has adownstream end tapering in a downstream direction which cooperates withthe intermediate section to vary the restriction to gas flow.
 2. Theexhaust system of claim 1 wherein the downstream end of the at least oneheader pipe extends into the exhaust chamber and the inner member ismounted for reciprocal movement on or in the downstream end.
 3. Theexhaust system of claim 1 wherein the engine has a plurality ofcylinders and an equal plurality of header pipes.
 4. An exhaust systemfor a four-stroke internal combustion engine having at least onecylinder and at least one header pipe receiving exhaust gases from theat least one cylinder, the exhaust system comprising:an exhaust chambercommunicating with the at least one header pipe and an exit port; ahollow inner member mounted at least partially within the exhaustchamber; said inner member movable in response to engine speed to varythe restriction provided to flow of gas through the exhaust chamber;said inner member has an upstream section, and a downstream section ofsmaller cross-sectional area, the upstream end including a taperedupstream section adjacent the downstream section, the upstream end andthe downstream end of the at least one header pipe cooperating to varythe restriction to gas flow.
 5. The exhaust system of claim 4 whereinthe inner member is mounted in the exit port.
 6. The exhaust system ofclaim 4 wherein the engine has a plurality of cylinders and an equalplurality of header pipes.
 7. An exhaust system for a four-strokeinternal combustion engine having at least one cylinder and at least oneheader pipe receiving exhaust gases from the at least one cylinder, theexhaust system comprising:an exhaust chamber communicating with the atleast one header pipe and an exit port; a hollow inner member mounted atleast partially within the exhaust chamber; said inner member movable inresponse to engine speed to vary the restriction provided to flow of gasthrough the exhaust chamber; said inner member has an upstream section,and a downstream section of smaller cross-sectional area, the upstreamend including a tapered upstream section adjacent the downstreamsection, the upstream end and the downstream end of the at least oneheader pipe cooperating to vary the restriction to gas flow; saiddownstream end of the at least one header pipe is tapered to becomplimentary to the upstream section.